Aromatic hydrocarbons (benzene, toluene, xylenes, etc.) serve as important precursors in the production of petrochemicals such as nylon, polyurethanes, polyesters, resins, and plasticizers. These petrochemicals are for the most part commodities, where the product conforms to a common specification with limited opportunity for differentiation.
A key to profitability in the petrochemical industry is to create and maintain a sustainable cost advantage over competitors. The cash cost of production is a critical factor, of which a significant portion is related to raw materials cost and utilities. While there is in principle no shortage of aromatics feed for production of petrochemicals, the cost of feed can vary significantly depending on the source of the feed. For example, a traditional way to recover high-purity aromatics is distillation, followed by liquid/liquid extraction, followed by further distillation. The liquid/liquid extraction step is expensive, due in part to the costs involved in purchase and recovery of the extraction solvent. A lower-cost alternative is to avoid the extraction step, and recover aromatics via distillation alone. However, distilled feeds contain co-boiling non-aromatics and other impurities that may impact yields and operations of the subsequent conversion process.
The separation of aromatics from non-aromatics is useful in upgrading aromatics-containing streams in petroleum refineries, such streams including naphtha streams, heavy catalytic naphtha streams, intermediate catalytic naphtha streams, light aromatic streams and reformate streams, and in chemical operations for the recovery of aromatics such as benzene, toluene, xylenes, naphthalene, etc.
The use of membranes to separate aromatics from saturates has long been pursued by the scientific and industrial community. Methods of membrane separation include hyperfiltration, also known as reverse osmosis (RO) in aqueous separations, pervaporation and perstraction. Pervaporation relies on vacuum on the permeate side to evaporate the permeate from the surface of the membrane and maintain the concentration gradient driving force which drives the separation process. In perstraction, the permeate molecules in the feed diffuse into the membrane film, migrate through the film and reemerge on the permeate side under the influence of a concentration gradient. A sweep flow of liquid or gas is used on the permeate side of the membrane to maintain the concentration gradient driving force. In contrast, hyperfiltration does not require the use of external forces on the permeate side of the membrane, but drives the separation through application of a pressure gradient across the membrane.
Early work with hyperfiltration or reverse osmosis, using cellulose acetate and polyethylene films showed some aromatics enrichment but at low membrane fluxes (Sourirajan, S., Reverse Osmosis, Academic Press, 1970). Subsequent studies demonstrated that the separation potential of a pervaporation system is much higher than that of RO (Rautenbach, R. and Albrecht, R., Journal of Membrane Science, 25, 1-54, 1985).
Membrane separation of aromatics from saturates has been the subject of numerous patents.
U.S. Pat. No. 3,370,102 discloses a general process for separating a feed into a permeate stream and a retentate stream and utilizes a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. The process can be used to separate a wide variety of mixtures including various petroleum fractions, naphthas, oils, hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene.
U.S. Pat. No. 2,958,656 discloses the separation of hydrocarbons by type, i.e., aromatic, unsaturated, saturated, by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.).
U.S. Pat. No. 2,930,754 discloses a method for separating hydrocarbons e.g., aromatic and/or olefins from gasoline boiling range mixtures, by the selective permeation of the aromatic through certain cellulose ester non-porous membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid.
U.S. Pat. No. 4,115,465 discloses the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation.
U.S. Pat. No. 4,929,358 discloses the use of polyurethane membranes for the separation of aromatics from non-aromatics. Permeation is suggested under pervaporation, perstraction, reverse osmosis, or dialysis conditions, however none of the experimental results reported in this patent were obtained under reverse osmosis conditions.
Polyimide membranes have been used for the separation of aromatics. U.S. Pat. No. 4,571,444 discloses the separation of alkylaromatics from aromatic solvents using a polyimide polymer membrane. The polyimide membrane of choice was an asymmetric polyimide polymer membrane prepared from a fully imidized, highly aromatic polyimide copolymer. Permeation was performed under reverse osmosis conditions.
U.S. Pat. No. 4,532,029 discloses the use of an asymmetric polyimide membrane for the separation of aromatics from lower aromatic middle distillate feeds. Permeation of the feeds in the presence of a light polar solvent, e.g., acetonitrile, was required to obtain permeates having a high aromatic content, i.e., greater than 86%.
U.S. Pat. No. 4,879,044 discloses a pervaporation process for separation of heavy catalytically cracked naphtha into a highly aromatic gasoline octane blending component and a low aromatic, high cetane distillate; U.S. Pat. Nos. 4,944,880, 4,946,594, 5,039,418 and 5,093,003 disclose improvements to membrane stability at high temperatures and U.S. Pat. Nos. 5,095,171 and 5,416,259 disclose improvements to the oxidative stability of membranes.
The majority of investigations for aromatic/non-aromatic separations have involved pervaporation or perstraction separation techniques. This is probably due to reports of prior literature that very high operational pressures are required in hyperfiltration to reach an equivalent performance achievable by pervaporation and perstraction processes. Unfortunately, pervaporation and perstraction separation systems are higher cost systems than hyperfiltration systems, due to expenses associated with vacuum, refrigeration and heat transfer systems.
Consequently, it is an object of this invention to provide an improved process for the separation of aromatic hydrocarbons from non-aromatic hydrocarbons in a feed stream using asymmetric polyimide membranes.
Another object of the present invention is to provide a method of improving the feed quality to an aromatic separation or aromatic conversion process by separating non-aromatic compounds from aromatic hydrocarbons in an aromatic hydrocarbon-containing feed stream by selectively permeating at least a portion of said aromatic hydrocarbons contained in said feed stream through a permselective membrane and diverting a primarily non-aromatic retentate out of the feed stream.
Other facets and advantages of the present invention will be apparent from the ensuing description and the appended claims.